Number of all possible topics is N, number of news on FP is 30. For example if N is 365, you would expect 70% chance that two separate items on FP mention the same topic.
Although, HN recurrence probably a bigger contributor. When a person gets reminded of some previous highly upvoted link and re-submits it. The highly upvoted link gets upvoted again and now you have two items on FP with similar keywords.
I agree, but the range of topics which may be popular at any given time is narrower and two stories about a single topic are likely to trend at similar times shrinking the sample space somewhat.
Number of all possible topics is N, number of news on FP is 30. For example if N is 365, you would expect 70% chance that two separate items on FP mention the same topic.
Although, HN recurrence probably a bigger contributor. When a person gets reminded of some previous highly upvoted link and re-submits it. The highly upvoted link gets upvoted again and now you have two items on FP with similar keywords.